test(uffd): add test_fault_all_handler_exit

The test verifies that if the fault-all handler exits, the VM is still
functional.  THe test is mostly relevant to Secret Free VMs.

Signed-off-by: Nikita Kalyazin <[email protected]>

Author: kalyazin

tests/integration_tests/functional/test_uffd.py

@@ -142,3 +142,31 @@ def test_malicious_handler(uvm_plain, snapshot):
             assert False, "Firecracker should freeze"
     except (TimeoutError, requests.exceptions.ReadTimeout):
         vm.uffd_handler.mark_killed()
+
+
+def test_fault_all_handler_exit(uvm_plain, snapshot):
+    """
+    Test that the VM is functional if the fault-all handler exits
+    after prepopulating the guest memory.
+    """
+    vm = uvm_plain
+    vm.memory_monitor = None
+    vm.spawn()
+    vm.restore_from_snapshot(snapshot, resume=True, uffd_handler_name="fault_all")
+
+    # Verify if the restored guest works.
+    vm.ssh.check_output("true")
+
+    # Kill the UFFD handler.
+    vm.uffd_handler.kill()
+
+    # Verify if the restored guest works after the handler exited.
+    #
+    # It is empirically known that invoking `ps` first time after snapshot restore
+    # will likely trigger an MMIO instruction lookup and emulation on x86
+    # that walks guest page tables via accessing them through the userspace mapping.
+    # On Secret Free VMs, we do not preinstall userspace mappings when prepopulating
+    # guest memory in the fault-all handler. If we fail to unregister all guest memory
+    # with UFFD on the handler exit, accessing the userspace mapping will trigger
+    # a UFFD notification that will never be handled.
+    vm.ssh.check_output("ps")